You can find something more complicated on MacFixIt, but I almost never have system problems with my Macs. I’ve owned them for years, fixed other people’s for years and rarely have problems due to system updates. Before you run another update on your machine, go download AppleJack and install it.
- Before you run that update, run Disk Utility and repair permissions on your internal drive.
- Run the update (NOTE: If it's an OS update, i.e. 10.4.x then run the Combo installer, which you can grab from VersionTracker, MacUpdate or directly from Apple)
- Wait for your computer to restart and don't touch anything until you see your desktop come up.
- Now, restart your machine again and after the screen goes blank, hold one finger on the apple key and another on the S key. You'll see a bunch of stuff on your screen that will make you think you've done something wrong. Don't worry.
- Type 'applejack auto restart' and don't touch your machine until yur machine restarts and you see your desktop again.